The manufacture of slow blowing cartridge fuses in such a small inner housing space described above has, heretofore, caused design problems to obtain consistent opening results. Because of this difficulty, prior to the present time there have been no commercially available, reliable slow blowing miniature fuses.
Commercial miniature fuses heretofore have been fast acting fuses. They are constructed with fine straight wires extending diagonally across opposite ends of the housings. The fuse wire ends bend around the housing ends where they are received between the housing and cup-shaped end caps applied thereover. This places the center point of the fuse wires where they are expected to open in what was believed to be the most ideal position in the center of the housing. Unfortunately, under overloads which do not immediately open the fuse, the fine fuse wire sometimes undesirably heats, expands and sags to a point where the fuse wire comes close to or touches the housing walls. The heat sinking effect of the nearby fuse housing increases the circuit current which is needed to open the fuse.
The inventor of the present invention made substantial efforts to develop a slow blowing miniature fuse which could be efficiently mass produced; but he failed until the present invention was developed. Slow blowing fuse elements typically comprise a fine fuse wire spirally wound axially along a non-rigid, somewhat flexible and resilient core of insulating material. Such fuse elements typically extend axially rather than diagonally along the fuse housing. An effort was made to center the fuse element so that the fuse wire is as far as possible from the walls of the housing, in accordance with the generally understood desirable practice to do so. (See FIGS. 2 and 3 for examples of these initial, unsuccessful efforts to design a slow blowing fuse). The spacing of the fuse element wire from the housing walls used in these efforts was much greater than that used in the case of the centered spiral wound fuse element shown in FIG. 7 of U.S. Pat. No. 4,460,887, which is not drawn to scale. (The specification of this patent confirms that the spirally wound fuse element shown in this FIG. 7 is spaced from the housing walls by stating that the diameter of the centered fuse element 11 is less than the diameter of the housing space.)
These initial efforts to make a slow-blowing fuse using a centered spirally wound fuse wire which is substantially spaced from the walls of the housing completely failed to produce a fuse capable of being mass produced because the manufacturing controls necessary to keep the fuse element centered would make the manufacturing method much too difficult and costly. When only modest process controls are used, as shown in FIGS. 1-3 of the present application, the spirally wound fuse elements assume different random positions with respect to the walls of the housing, resulting in variations in the blowing current values which went beyond acceptable tolerances.